1. Field of the Invention
The present invention relates in general to integrated circuit (IC) manufacturing and, more specifically, to methods in IC manufacturing processes for identifying and redirecting IC's mis-processed during their manufacture.
2. State of the Art
As shown in FIG. 1, a typical process 10 for manufacturing very small electronic circuits referred to as "Integrated Circuits" (IC's) begins with the IC's being formed or "fabricated" on the surface of a wafer 12 of semiconductor material, such as silicon. Once fabricated, IC's are electronically probed to determine whether they are functional (i.e., "good") or nonfunctional (i.e., "bad"), and a computer then stores an electronic wafer map 14 of the wafer 12 identifying the locations of the good and bad IC's on the wafer 12.
After being probed, IC's are sawed from their wafer 12 into discrete IC dice or "chips" using high-speed precision dicing equipment. IC dice identified as good by their wafer map 14 are then each "picked" by automated equipment from their sawed wafer 12 and "placed" on an epoxy coated bonding site of a lead frame, while IC dice identified as bad are discarded into a scrap bin 16. The epoxy attaching the good IC dice to their lead frames is then allowed to cure, and the attached dice are wire bonded to their lead frames using high speed bonding equipment. At this point in the process 10, the lead frames of IC dice are still connected to other lead frames.
Once wire bonded, IC dice and their lead frames are formed into IC packages using a hot thermosetting plastic encapsulant injected into a mold. Leads of the lead frames project from the IC packages after encapsulation, and these leads are dipped in a cleansing chemical bath in a process referred to as "de-flash." After de-flash, IC packages are cured to set their plastic encapsulant, and their projecting leads are then electroplated with a lead/tin finish.
After lead finishing, connections between the lead frames of different IC packages are cut to "singulate" the IC packages into discrete IC devices. Discrete devices are then tested in a simple electronic test that checks for "opens" (i.e., no connection) within the devices where connections should exist and "shorts" (i.e., a connection) where connections should not exist. Devices that fail the opens/shorts test are discarded into the scrap bin 16, and devices that pass proceed to extensive back-end test procedures where they are tested for functionality and operability before being shipped to customers.
On occasion, bad IC dice are accidentally picked from a sawed wafer 12 for subsequent assembly and back-end testing as described above. This can happen, for example, because a human, software, or electronic error causes the automated pick and place equipment described above to access the wrong wafer map 14 for a wafer 12. It can also happen because of a misalignment, referred to as a "registration" error, between the automated pick and place equipment and a wafer 12. In either case, such accidents typically are not detected until the bad IC dice undergo at least some back-end testing and, as a result, waste back-end testing resources. Therefore, there is a need in the art for a method of identifying and discarding accidentally assembled IC dice before the dice undergo back-end testing procedures.
As described in U.S. Pat. Nos. 5,301,143, 5,294,812, and 5,103,166, some methods have been devised to electronically identify IC dice. Such methods take place "off" the manufacturing line, and involve the use of electrically retrievable identification (ID) codes, such as so-called "fuse ID's," programmed into individual IC dice to identify the dice. The programming of a fuse ID typically involves selectively blowing an arrangement of fuses or anti-fuses in an IC die so that when the fuses or anti-fuses are accessed, they output a selected ID code. Unfortunately, none of these methods addresses the problem of identifying and discarding accidentally assembled IC dice "on" a manufacturing line.